Measuring ultra-low impedances (20 micro-ohm) is a challenge for any power distribution network (PDN) design engineer. The 2-port shunt-through measurement is the standard method for measuring milliohm impedances up to very high frequencies (GHz). Unfortunately, this measurement includes an undesirable ground loop related to the instrument grounds and test setup cabling. The ground loop introduces significant errors if proper care is not taken. This application note shows how to measure as low as 20 μΩ using the Picotest J2102A as a ground loop breaker.
Controlled impedance printed circuit boards (PCBs) often include a measurement “coupon”, which typically includes sample traces, 6 inches long and constructed as part of the printed circuit board panel. They are measured to assure the PCB stack-up and the accuracy of the PCB transmission lines. These coupons, as well as the actual PCB signal traces, interconnects and cables are typically measured using a Time Domain Reflectometer (TDR). A TDR is generally a large, expensive instrument that includes a high-speed edge pulse and a sampling oscilloscope.
The topic of clock jitter performance seems to be a current focus of clock, ADC, and power supply manufacturers. The reasoning is clear; clock jitter interferes with the performance of digital circuits including high speed ADCs. High speed clocks can be quite sensitive to the “cleanliness” of the power they receive, though quantifying the relationship takes some effort.
Low power, high performance circuits are often plagued by power supply related issues. This common occurrence is frequently due to mythical (or misapplied) rules-of-thumb.